Florida Sen. Marco Rubio told Senate Republican Leadership on Thursday that he intends to vote against the massive tax bill barreling through Congress if the child tax credit isn’t expanded, a potential major blow in President Donald Trump’s desire to pass a tax overhaul by Christmas.

If the bill isn’t changed and Rubio votes against the plan, there would be no room for additional Republican dissension as the GOP only holds 52 of 100 Senate seats. Tennessee Sen. Bob Corker already announced that he would vote against the plan due to concerns on the federal deficit, leaving Republicans with only 51 votes.

Vice President Mike Pence would break a tie if the GOP has 50 votes.

Rubio and Sen. Mike Lee, R-Utah, proposed a change to make the child tax credit fully refundable as a way to help low-income families, but that plan was opposed by Sen. Majority Leader Mitch McConnell and GOP leadership and the measure failed.

Despite the failure of their proposed changed, Rubio and Lee voted for the initial tax bill that passed the Senate two weeks ago with 51 votes.

Rubio has made it clear he wants an expanded child tax credit for months, and President Donald Trump hinted at an expansion on Wednesday saying, “You'll hear the numbers very soon but they're even larger than anticipated.”

The child tax credit reduces some families' tax bill for every child they have under the age of 17.

Rubio has repeatedly said he would vote against a tax plan that does not sufficiently benefit the middle class, though he has previously stopped short of threatening to vote against the final plan due to the child tax credit until now.